Humans aren't the only primates when it comes paying for sexual services, for a new study has found that monkeys too fall in the category.
Michael Gumert, a researcher at Division of Psychology at Nanyang Technological University, Singapore, has shown that male longtailed macaques exchange grooming for the right to mate with females whose fur they cleaned.
The study presents the first proof that a "social market" influences sexual interaction in a non-human primate.
"I found that the amount of grooming a male performs on a female during a sexual interaction is related to the supply/demand ratio of females per male around the male-female pair at the time of the grooming," Discovery News quoted Gumert, as saying.
Gumert found that male monkeys, particularly lower status ones, have to groom more to get more sex when fewer females are present.
Grooming in macaques, which involves using the teeth and hands to pick through the fur of the recipient to remove dirt, tangles and parasites, often leads to sexual excitement in monkeys, mainly the males.
Hence, many scientists have theorized that the activity evolved into foreplay in humans.
For the study, Gumert studied a wild population of longtailed macaques at Tanjung Puting National Park, Indonesia, from 2003 to 2005.
He documented 243 male-to-female grooming sessions during this period, most of which were mean for females who were open to mating.
The "grooming before sex" sessions lasted from a few seconds to a half hour or more, with the durations often linked to either the number of prospective other partners or to the status of the groomer or recipient.
According to Gumert, "rank does not remove the market, it only skews it."
"Powerful individuals can take more and give less than low-ranked individuals can," he said, indicating that such corruption of the fair trade model seems to be an innate aspect of primate social life that can connect to everything from monkey sex to human politics.
He said that high-ranking females could also distort the system because, in the case of macaques, they command more attention before they approve to mate. Since males often have their work cut out for them, they also try to first "flirt" with females, using facial gestures before they advance.
"Being anthropomorphic, this may be like winking or smiling. The male bows and bobs his head, raises the eyebrows and smacks his lips at the female," Gumert said.
He also discovered that though females groom males now and then, this behaviour doesn't appear to be linked to sex.
Frans de Waal, a professor of psychology and director of the Living Links Center at the Yerkes Primate Center at Emory University in Atlanta, said the new study is "very well done and nicely applies the biological market concept to something new -- exchange of grooming for sex, or sex for grooming."
"We all know that primate males often do a bit of grooming before they mate with females, and that they groom very little if the female isn't fertile, but it is good to see such a thorough, quantified account of it," he added.
The study has been accepted for publication in the journal Animal Behaviour.